1. Field of the Invention
The present invention relates to a spot welding robot controlling method and controlling apparatus. More particularly, the present invention relates to a method of controlling a spot welding robot with a spot welding gun which is driven by a fluid pressure such as pneumatics, oil hydraulics, etc., and an apparatus for embodying the same.
2. Description of the Related Art
In the related art, as shown in FIG. 3, a spot welding gun (sometimes referred to as a "gun" hereinafter) 2 is employed in a spot welding robot (sometimes referred to as a "robot" hereinafter) 60. The spot welding gun is equipped with a pair of gun tips 1 which are operated by a fluid pressure such as pneumatics, oil hydraulics, etc. By using the spot welding robot 60 with such spot welding gun 2, a spot welding is carried out by initially teaching spot welding points according to a teaching operation, then closing the spot welding gun 2 at points taught according to a reproducing operation by a robot controller 3, and then supplying a current to the gun tips 1 from a welding machine 4. Normally, a signal for closing the gun 2 (gun pressurizing signal) is output when axial coincidence of the spot welding robot 60 (current position axial coincidence) is achieved at a position which is taught previously. Also, a signal for supplying a current to the gun tips 1 (welding command signal) is output when the gun 2 has been closed, i.e., a pressurizing operation of the gun 2 has been completed (at the gun pressurization completing time point).
In this manner, according to the spot welding robot 60 in the related art, since the gun pressurizing signal is output at a time point when axial coincidence in a current position (current position axial coincidence) of the spot welding robot 60 is achieved at taught points, a time until the gun 2 has been closed is a dead time.
Therefore, various proposals have been made in order to reduce this dead time in the related art.
For example, such a proposal has been made that the gun pressurizing signal is output prior to axial coincidence where a command position (command position axial coincidence) of the spot welding robot 60 is achieved. However, according to a method in this proposal, it has been apparent that such a phenomenon is caused depending upon spot welding points that either the pressurization to the gun has been completed after the current position axial coincidence, or conversely the pressurization to the gun has been completed prior to the current position axial coincidence. The reason for this phenomenon may be supposed as follows. That is, the equal time is set to all spot welding points to output the gun pressurizing signal, nevertheless a time required from the command position axial coincidence to the current position axial coincidence is varied depending upon differences in moving routes, welding positions, etc. of the spot welding robot 60. Where the wording "command position axial coincidence" signifies that the command position coincides with the welding point, and the wording "current position axial coincidence" signifies that actual position of the spot welding robot 60 coincides with the welding point. As a result, according to a method by this proposal, it is impossible to complete the pressurization to the spot welding gun 2 at a time point when the current position axial coincidence is achieved at all spot welding points.
In Japanese Patent Application Publication (KOKAI) Hei 6-23561, there has been proposed a spot welding robot controlling method wherein a time ranging from a time point when the pressurization command is output from the robot controller to the spot welding gun to a time point when the pressurization to the spot welding gun is completed is measured in advance and then stored in the robot controller, then a moving time needed to position the spot welding robot is calculated every time when a positioning operation for the spot welding is commenced, then difference between the moving time and the above-mentioned time is calculated, and then the pressurization command is output to the gun at a time point when the above-mentioned time difference has been passed after starting of the positioning operation. However, the moving time of the spot welding robot is derived by virtue of calculation in this proposal, i.e., the time calculated based on an arrival time of the command position is utilized as the moving time, but the actual motion of the spot welding robot is started in answer to the command position with slight delay due to delay in a servo system, etc. Therefore, there has been a problem such that, if the starting time for the pressurization to the gun is set by using the moving time which is calculated based on the command position, the pressurization to the gun is completed before the spot welding robot reaches to the taught position. In addition, there has been the problem that, since the pressurizing time is set as an average value which is calculated after the pressurizing time has been measured by pressurizing the spot welding gun several times, such pressurizing time cannot respond to variation in time caused by the gun positions, the moving routes of the gun, abrasion of the gun tips, or the like. Moreover, there has been the problem that, since this measurement must be executed in an off-line mode, measuring operations become troublesome and complicated.
In Japanese Patent Application Publication (KOKAI) Hei 6-47562, there has been proposed a spot welding method wherein, in the situation that a gun ON command is issued during a robot job in the spot welding robot, the gun ON command is output to the spot welding power supply only when a difference between a target value instructed by the robot and a current position is within a certain range and also an actual speed of the robot is less than a predetermined value. However, although the gun ON command is output only when the difference between the target value and the current position is within the certain range and the actual speed of the robot is less than the predetermined value, it is not checked in this proposal whether or not a current position of the robot has reached the target value. Therefore, there has been such a problem that it is not clear whether or not the pressurization to the gun has been completed at a time point when the current position of the robot reaches to the target value.
In Japanese Patent Application Publication (KOKAI) Hei 7-136775, there has been proposed a spot welding equipment which comprises a robot; a welding gun attached to the robot, for applying the welding force to a welded workpiece via motion of the electrode tips caused by a pressurizing air cylinder; an electromagnetic switching valve connected to the pressurizing air cylinder; a gun pressurization commanding means for outputting a pressurizing operation command to the electromagnetic switching valve based on a welding force applying characteristic of a welding gun generated by the pressurizing air cylinder before the welding gun reaches a welding position such that both the arrival of the welding gun of the robot to the welding position and the completion of the gun pressurization via the electrode tips is achieved simultaneously; and a gun pressurization release commanding means for outputting a gun pressurization releasing command to the electromagnetic switching valve based on a gun pressurization releasing characteristic of the welding gun by the pressurizing air cylinder after supply of the current to the welded workpiece has been completed such that both the start of motion of the welding gun of the robot to the succeeding welding position and the gun pressurization releasing operation via the electrode tips is achieved simultaneously. However, according to the spot welding equipment in this proposal, characteristic such as a time extending from the start of the pressurization to the completion of the gun pressurization, etc. have been derived previously based on experiments. Therefore, both the arrival of the welding gun to the welding position and the completion of the pressurization to the welding gun cannot always be achieved at the same time in the actual spot welding operation. This is because, as described above, the time required from the start of the pressurization to the welding gun to the end of the gun pressurization is varied due to the gun positions, abrasion of the gun tips, or the like in the actual spot welding state. Also, in the spot welding equipment according to this proposal, the pressurization command for the gun is issued at a predetermined time before the welding gun reaches the welding position. However, a time period during which the robot is moved from the current position to the welding position must be known to issue such pressurization command for the gun, but no mention about this respect has been made.